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Purdue UniversityPurdue e-PubsInternational Refrigeration and
Air ConditioningConference School of Mechanical Engineering
2006
Experimental Research and CFD Simulation onMicrochannel
Evaporator Header to Improve HeatExchanger EfficiencyZhihai Gordon
DongAmerican Power Conversion Corp.
John BeanAmerican Power Conversion Corp.
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Dong, Zhihai Gordon and Bean, John, "Experimental Research and
CFD Simulation on Microchannel Evaporator Header to ImproveHeat
Exchanger Efficiency" (2006). International Refrigeration and Air
Conditioning Conference. Paper
753.http://docs.lib.purdue.edu/iracc/753
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EXPERIMENTAL RESEARCH AND CFD SIMULATION ON MICROCHANNEL
EVAPORATOR HEADER TO IMPROVE HEAT EXCHANGER EFFICIENCY
Zhihai Gordon Dong1, John Bean Jr.2
1Research and Development, NetworkAir Dept.,
American Power Conversion Corp. 801 Corporate Center Dr., St.
Charles, MO, US, 63304
Email: [email protected]
2Research and Development, NetworkAir Dept., American Power
Conversion Corp.
801 Corporate Center Dr., St. Charles, MO, US, 63304 Email:
[email protected]
ABSTRACT Microchannel evaporator performs superior heat transfer
efficiency and capacity at compact size comparing with conventional
tube-fin evaporator. Design an appropriate coil header assembly is
one of major tasks, which could affect the desired heat exchanger
efficiency and capacity. An experimental investigation of three
coil inlet header configurations, which are single, dual and
distributor inlet headers, is conducted in the paper. A practical
evaluation method, by means of measuring air temperature
differential across coil to evaluate refrigerant distribution
uniformity in coil header, is introduced. CFD models are also
generated to simulate refrigerant liquid flow contours of these
three inlet header configurations. Finalize the research, the
distributor header configuration appears the most uniform
distribution in conjunction with symmetrical flow distribution. The
dual inlet header configuration also significantly improves
distribution uniformness and symmetry comparing with the single
inlet header configuration.
1. INTRODUCTION
1.1 Introduction of microchannel heat exchanger A microchannel
heat exchanger appears advanced characters comparing with a
conventional round tube-fin heat exchanger. Refrigerant flows
through multiple microchannel flat tubes, which contain
microchannels ports rather than single wall round tubes. This
significantly enhances the heat transfer area and overall film
coefficient of the microchannel heat exchanger. High efficiency of
the microchannel heat exchanger enables the heat exchanger to be
made in smaller size, light weight, and yet has the same
performance as a conventional round tube-fin heat exchanger.
Refrigerant charge of the cooling system is also reduced. Due to
many advantages of microchannel heat exchanger, it has been widely
applied in residential air conditioning and automotive industry.
However, comparing with a round tube-fin heat exchanger,
microchannels ports causes higher refrigerant pressure drop across
heat exchanger, it might be an issue to some systems. Condensation
and defrost of microchannel coil are also major issues for
refrigeration and air conditioning applications. The uniformity of
refrigerant distribution within coil inlet header is another major
issue for microchannel heat exchanger. A properly designed coil
inlet header should uniformly distribute refrigerant into
microchannels, and refrigerant would perform sufficient heat
transfer inside microchannels tubes. Eventually, coil cooling
efficiency is optimized and capacity is maximized. On the contrary,
a defective coil header assembly could cause uneven refrigerant
flow inside microchannels and reduce coil cooling efficiency and
capacity. In the worst situation, the defective header design might
cause the danger of gas and liquid flow separation, which could
significantly damage coil heat transfer performance. Therefore, to
achieve uniform refrigerant distribution, the coil header
configuration and orientation becomes to be a very important design
task.
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1.2 A refrigerant liquid pumping cooling system for data center
air conditioning application The major duty of a data center air
conditioning system is to remove sensible heat, which is generated
by electronic equipments. Condensation is controlled and minimized
in the system. Ideally, sensible heat ratio of this air
conditioning system would equal to one. Modern servers are
integrated into limit space. Heat load density in the space is
high. A compact air conditioning system with high cooling
efficiency and capacity needs to be developed to remove this high
density heat flux. A R134a liquid pumping cooling system is
introduced to this application. The cycle of system circuit is
shown in Figure (1).In the diagram, subcooled liquid R134a enters
pump intake at state point (1) to process adiabatic compression.
Further subcooled liquid R134a leaves pump discharge port at state
point (2). State point (2) to (3) presents liquid R134a flows
through liquid line and reaches to inlet of evaporator. From state
point (3) to (4), subcooled liquid is vaporized in evaporator.
Slight superheated gas leaves evaporator at state point (4) and
return back to condenser inlet at state point (5) via a suction
line. R134a gas rejects heat to chilled water in condenser, and it
becomes subcooled liquid. The subcooled liquid returns back to pump
intake at state point (1) and cycle starts over again.
Figure (1) R134a liquid pumping cooling system cycle
1.3 Microchannel heat exchanger in the refrigerant liquid
pumping cooling system The microchannel coil is horizontally
installed in space above two symmetrical heat loads. Upon means of
control coil evaporating temperature, condensation is avoided to be
generated on coil. However, due to subcooled liquid R134a flows
into the large dimensional coil inlet header (approx 23 long), a
proper coil inlet header configuration becomes important issue to
accomplish refrigerant uniform distribution into microchannels. 1.4
Three configurations of microchannel coil inlet header To optimize
coil inlet header assembly, three configurations of microchannel
coil inlet header are conducted and investigated in this paper.
These three configurations - single, dual and distributor inlet
headers are illustrated in Figure (2).
(a) Single inlet header (b) Dual inlet header (c) Distributor
inlet header
Figure (2) Three configurations of microchannel coil inlet
header
The single inlet header is constructed by a single round tube,
which is simply welded to center of coil inlet manifold in
perpendicular direction. The dual inlet header employs a secondary
header, which is jointed to the coil inlet manifold via two round
tubes at and length of coil inlet manifold. Main liquid entry line
is connected to center of the secondary header. The entire dual
inlet header assembly is furnace welded. Refrigerant liquid flows
into the secondary header first. It is subsequently divided into
two braches by the two round interjunction tubes, and flows into
coil inlet manifold. The third configuration is the distributor
inlet header. Refrigerant liquid flow is evenly divided inside
hollow conical portion of the distributor. A group of small
capillary tubes are connected to the
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distributor to pick up the branched refrigerant flow. They
deliver refrigerant liquid into coil inlet manifold at even
portions. 2. THE APPROACH TO EVALUATE REFRIGERANT DISTRIBUTION
UNIFORMITY
INSIDE COIL INLET HEADER 2.1 Heat transfer and thermodynamic
process inside microchannel tube To simplify heat transfer and
thermodynamic analysis inside microchannel ports, we consider the
group of microchannel ports as one single microchannel tube. The
process is illustrated in Figure (3).
Figure (3) Refrigerant heat transfer and thermodynamic process
inside microchannel tube (Not to scale)
As shown in Figure (3), refrigerant flow inside microchannel
tube is composed by three segments, which are liquid sensible
heating, evaporating, and gas sensible heating segments. The local
convective heat transfer flow rate Qhx is calculated in Equation
(1).
Qhx = 0A h thx dA (1) The convective heat transfer coefficient h
is strong dependent upon the refrigerants physical properties,
situation and Reynolds number. It is much higher within evaporating
(two-phase flow) segment due to bubble generation and other major
factors. Heat transfer flux appears extremely active within this
area, while the heat transfer density is much lower within
subcooled and superheated segments (one-phase flow). There is no
phase change within liquid sensible heating and gas sensible
heating segments. Refrigerant sensible heat flow rate in these
areas is donated in Equation (2). Qr = mr Cpr tr (2) Refrigerant
performs phase change in evaporating area, the latent heat flow
rate of refrigerant in the area is shown in Equation (3). Qr = mr r
(3) R134a latent heat r is almost 20 times larger than its isobaric
specific heat Cpr within this heat exchanger working range. Heat
transfer flux in microchannel is primarily contributed by
refrigerants latent heat. Airflow vertically flows through
microchannel tube, and it rejects sensible heat to refrigerant. The
airflow temperature differential across microchannel tube ta is
expressed in Equation (4).
ta =trtn tsply = Qa / (ma Cpa) (4) Air isobaric specific heat
Cpa approximately equals to 1.01(kJ/kg- K). When air mass flow rate
ma is uniformly remained as constant, airflow temperature
differential crossing coil ta is direct proportional as function of
air sensible heat flow rate Qa. 2.2 The approach to evaluate
distribution uniformity of refrigerant flow inside coil inlet
header Viscous fluid convective heat exchange is difficult to
analyze and predict. In practice, many of this type of heat
transfer analysis are treated empirically. It is difficult to
inspect refrigerant flow rate distribution inside coil inlet header
directly. Instead of direct pursue flow distribution contour inside
coil header, a thermodynamic and heat transfer analysis in
microchannel portion could be a strong indicator of refrigerant
distribution status inside coil inlet header. It is a practical way
to evaluate the coil inlet header distribution uniformity.
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As been described before, within the subcooled liquid and
superheated gas segments, heat transfer coefficient h and heat
transfer flow rate Qhx remain at relative lower level. Airflow
temperature difference across these microchannel segments ta
appears relative small. Within evaporating segment, although the
heat transfer coefficient h is various, ta is relative much higher
as result of strong heat transfer flux. Airflow temperature
differential ta across microchannel appears significantly larger.
Therefore, airflow temperature differential across microchannel ta
directly indicates refrigerant status inside microchannel tube.
When ta is significant higher, refrigerant is in two-phase status,
and it performs strong heat transfer inside microchannel tube. When
ta is lower, it indicates that refrigerant is either in liquid or
gas state inside microchannel tube. Refrigerant mass flow rate is
various between microchannel tubes due to refrigerants uneven
distribution inside coil header. The larger mass flow inside
microchannel is, the further behind transient locations of segment
will be. The gradient of segment boundary (transient) locations
forms two curves of parabola (refer to Figure (9)) on microchannel
coil surface. These two parabolas divide coil into three segments
of liquid sensible heating, evaporating and gas sensible heating. A
degenerate parabola indicates a better uniformness of refrigerant
flow distribution inside coil inlet header. On the contrary, an
evolvable parabola donates the worse refrigerant flow distribution
inside coil inlet header.
3. EXPERIMENTAL AND DATA ACQUISITION SYSTEM 3.1 The R134a
experimental cooling system and components The experimental system
schematic is shown in Figure (4). Refrigerant liquid pump (A) is
controlled by a variable speed controller (B). A motorized
three-way valve is used to modulate building chiller water flow,
which flows through a brazed plate condenser (E). The combination
of motorized three-way valve and variable speed liquid pump
maintain the desired evaporating temperature and superheat (state
point (4)). Axial fan array (D) delivers airflow through the
evaporator (C) at a desired constant airflow rate. A test chamber
is conducted to simulate the natural work environment. Three
microchannel coils (C), which are assembled with three
configurations of inlet header, is horizontally installed in space
above symmetrical heat loads. Figure (5) shows the
installation.
Figure (4) Schematic of experimental R134a liquid
pumping system
Figure (5) Microchannel coil assembly is horizontally installed
in space above symmetrical heat loads
3.2 Data acquisition instrumentation An Agilent 34970 data
logger is used in the data acquisition system. Capacitance type
pressure transducers are installed at evaporator inlet and outlet
to measure R134a entering and leaving pressure. Thermistors are
mounted to the same location to measure R134a entering and leaving
coil temperature. The evaporator inlet subcooling and outlet
superheating temperatures are computed by combination of these
pressure and temperature readings.Ideally, ta across each
microchannel tube should be measured to locate these segment
transient locations, which was previously introduced in this paper.
However, this will require an extremely large number of temperature
measure nodes on coil both sides. To simplify measurements, the
coil surface is divided into nine of even zones on coil each side.
As shown in Figure (6), an array of 3 x 3 temperature measure nodes
(located at center of each zone)
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on coil each side is arranged in this experiment. Thermistors
are installed at these discrete points to measure return air
temperature (Trtn at bottom surface of coil) and supply air
temperature (Tsply at top surface of coil).
Figure (6) Coil zones (nine on coil each side) and temperature
measure nodes layout
4. EXPERIMENT RESULT AND DISCUSSION
4.1 Test result and airflow temperature differential across coil
Air mass flow rate is maintained at a constant during tests. R134a
evaporating temperature is maintained at 63F, and superheat at coil
outlet is controlled at 3.9oC (7oF). Subcooling at coil inlet stays
at 4.4oC (8oF). Average readings of supply air temperatures Tsply
and return air temperatures Trtn (in parentheses) at discrete
center points of these zones are listed in Figure (7). Coils are
presented as 2D top views in this Figure.
Figure (7) Tsply and Trtn (in parentheses) measurements at coil
each zone
Figure (8) Air temperature differential ta across coil each
zone
As defined in Equation (4), airflow temperature differential
across microchannel is calculated at ta =trtn tsply. Figure (8)
donates the ta across each coil zone.
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4.2 Analysis of refrigerant distribution uniformity inside coil
inlet header In general, middle portion (zone4, zone5 and zone6) of
coil should be mainly occupied by refrigerant evaporation. Here we
choose zone6 as the benchmark zone, and assume this zone is fully
fulfilled with refrigerant two-phase mixture. Airflow temperature
differential across each coil zones could be illustrated as
percentage ratio to ta of zone6. Each percentage ratio donates
weight of refrigerant evaporation portion within that zone. By
means of converting the percentage ratio into linear scale,
appropriate trendlines across these points can be drawn. These
curves are in shape of parabola as shown in Figure (9). Although
each parabola is not physical segment transient location
(boundary), it is excellent substitute (trendline) to represent the
segment transients location and tendency. Each coil is divided into
three segments by two parabolas. These three areas are approx.
liquid sensible heating, evaporating, and gas sensible heating
segments of that coil.
Figure (9) Weight ratio of evaporation within each zone and coil
segment trendlines on coil surface
Analyze these trendlines in Figure (9), the most evolvable
parabola along the axial direction appears on single inlet header
coil. It indicates the single inlet header coil performs the worst
refrigerant flow distribution among these three coils. On the
contrary, the distributor inlet coil header appears the most
uniform refrigerant flow distribution. The dual inlet coil header
significantly improves refrigerant flow distribution comparing with
the single inlet coil header. Another phenomenon is the inclination
on parabola symmetry. The inclination of turbulent flow has been
minimized prior to enter into coil inlet header. Therefore, the
inclination inside coil header is mainly caused by asymmetrical
geometric factors of inlet header and surrounding area. The
parabola for the single inlet header coil appears the most
asymmetrical among the three types of coil. This is because that
refrigerant liquid flows into the single inlet header via single
tube, any asymmetric factor of the inlet tube and surround flow
path could cause an obvious inclination on liquid flow direction.
Furthermore, it influences the symmetry of distribution. In the
case of dual inlet header, refrigerant liquid is divided into two
branches to flow into the coil header manifold. This reduces the
influence by asymmetric factors of each single branch. 4.3
Experiment result accuracy The experiment was conducted inside a
test chamber to simulate the real working environment. Return air
temperature was not perfect uniformed at these discrete points on
the coil air return surface. This affected air temperature
differential ta crossing coil at certain level. However,
accomplishing with coil header improvement, the gradient of ta is
uniformed significantly. This donates that a certain level of
return air temperature gradient did not affect the experiment
conclusion.
5. NUMERICAL STUDY OF THREE INLET HEADER CONFIGURATIONS 5.1 CFD
model characterization and assumption A CFDesign software is
utilized to algorithmically compute the refrigerant liquid flow
contours inside these three types of inlet header. Refrigerant flow
is assumed as a single-phase, incompressible viscous turbulent flow
at steady state, and process is adiabatic. The CFD model geometry
of the header and coil assembly is characterized as a 3D smooth
internal pipe flow. Boundary condition is constrained at 1 gpm of
volume flow rate at coil inlet and 0 Pa pressure at coil exit. Coil
material is given as aluminum, and fluid type is R134a. The element
mesh size is set at 0.5, with total element number of approx. 10k.
Total number of compute iterations is set at (100).
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5.2 Compute result and velocity magnitude contours The CFD
computed velocity magnitude contours of refrigerant flow inside
coil inlet header is showed in Figure (10). Obviously, the single
inlet header concentratedly distributes refrigerant into middle
portion of microchannel coil; the dual inlet header significantly
improves the refrigerant distribution uniformity inside inlet coil
header; and the distributor inlet header performs the most
uniformed refrigerant distribution.
(a) Single inlet header (b) Dual inlet header (c) Distributor
inlet header
Figures (10) Refrigerant velocity magnitude contours inside
inlet coil header 5.3 CFD analysis of inclination on refrigerant
distribution symmetry by inlet header tube slope
(a) Dual inlet header with no tube slope (b) Dual inlet header
with 10o tube slope
Figure (11): Symmetry inclination of refrigerant velocity
magnitude contours by inlet header tube slope
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As previous stated in the paper, inclination of turbulent flow
and many asymmetric geometric factors of inlet header and
surrounding flow path could cause symmetry inclination on header
liquid distribution. The slope of inlet header tube is one of these
geometric factors. To demo the inclination, a 10-degree inlet tube
slope angle is added to the dual inlet coil header model. The
compute result is illustrated in Figure (11). As showed in the
Figure, refrigerant velocity magnitude axis center is moved
downwards due to the 10o of inlet tube slope.
7. CONCLUSIONS An appropriate coil inlet header assembly design
is one of the essential tasks to improve the microchannel heat
exchanger heat transfer efficiency and maximize the cooling
capacity. Among these three inlet header configurations, the
distributor header performs the most uniform refrigerant liquid
flow distribution conjunction with good flow symmetric contour. The
dual inlet configuration significantly improves refrigerant flow
distribution uniformity and symmetry.
NOMENCLATURE
Symbol Description Unit Subscripts A Heat transfer area (m2) a
Air C Specific heat capacity (J/kg- oC) hx Heat exchanger h
Convective heat transfer coefficient (W/m2- oC) p Isobaric m Mass
flow rate (kg/s) r Refrigerant t Temperature (oC) rtn Air return Q
Heat flow rate (W) sply Air supply Differential Latent heat of
vaporization (J/kg)
REFERENCES Bergles, A.E., Lienhard V, J. H., Kendall, G.E.,
Griffith, P., 2003, Boiling and Evaporation in Small Diameter
Channels, Heat Transfer Eng., vol. 24, no. 1, p. 18 -40. Kakac, S.,
Liu, H., 2002, Heat Exchangers Selection, Rating and Thermal
Design,V2., CRC Press, US, 501 p. Kim, J., Groll, 2003, Performance
and Reliability of Microchannel Heat Exchangers in Unitary
Equipment, The Ray W. Herrick Lab., Purdue Univ., 10 p. Zhao, Y.,
2001, Previous Studies on Microchannel Heat Transfer, Chapter 2.2,
In: Ohadi, M.M., Radermacher, R., Microchannel Heat Exchangers with
Carbon Dioxide, Maryland Univ., p. 11-17
ACKNOWLEDGEMENT
Purdue UniversityPurdue e-Pubs2006
Experimental Research and CFD Simulation on Microchannel
Evaporator Header to Improve Heat Exchanger EfficiencyZhihai Gordon
DongJohn Bean